As semiconductor devices become increasingly complex, so does the time required to properly test these devices. Considering the high volume production of some semiconductor devices, such an increase in testing time and the corresponding testing cost can significantly increase the cost of the final product. Additionally, semiconductor devices are often manufactured in more than one location, often by different subcontractors or fabrication companies. This often results in different testing specifications between the sites because different tests and testing procedures may be used. Consequently, minimizing the testing time and standardizing the testing specifications are two challenges for any wafer-testing process.
Wafer-testing is physically conducted by a device known as prober. The prober holds a probe card, on which needle-like probe pins are aligned in a configuration that corresponds to a specific semiconductor device. Through the probe pins, the prober makes electrical contact with the dice on a wafer and performs various test patterns, which are also called test programs. Different semiconductor devices require different testing programs. A die may pass all or just some of the test programs, and based on such testing results, the die is then categorized as good, bad, or even some other classifications. The process for wafer testing has already been addressed by prior art such as the Taiwanese patent issued to Jeng et al. (TW 516149).
During the wafer-testing process, high failure rate at specific die locations or low overall yield rate are often indications that there might be a defect in the wafer fabrication process. The defect may locate either in the production stage or in the testing stage of the wafer manufacturing process. There are several possible defects that can occur in the testing stage of the process. One possible defect is the faulty design of the test programs. Another possible defect is the mechanical failure of testing equipments. The probe pins on the probe card may be misaligned or the prober's hold of the probe card may become loose. The prober itself may also suffer mechanical failures such as leaking hydraulic fluid or coolant. Often times, these problems arise simply due to the wear and tear of the testing process.
Moreover, an existing defect may come with a new piece of testing equipment. Improper maintenance of the testing equipments may also cause an equipment defect. Thus, it is important to ensure that there is no defect before starting up a new testing equipment or restarting an existing testing equipment that has been serviced.
A technique known as wafer correlation has been used to diagnose the existence and location of a defect. Correlation involves first choosing a test wafer and determining the locations of the good and bad dice on the correlation wafer. The test wafer is often called a correlation wafer. One way to make such determination is to test the correlation wafer using testing equipments that are working properly. Using the testing results of the correlation wafer, a reference map recording the location of the good and bad dice is generated and stored as reference data. Before startup or restart, or after a defect is suspected, an operator would test the same correlation wafer to again determine the locations of the good and bad dice. The operator then applies a set of correlation criteria to compare the test results with the previously-determined reference data and determine the number of matching dice. The testing results and reference data correlate only if the number of matching dice exceeds a threshold number. If the testing results correlate with the previously-determined reference data, then there is either no defect, or a defect is located in the production stage. If the test results do not correlate with the previously-determined reference data, then a defect is likely located in the testing stage.
Although wafer correlation is an important diagnostic technique, it has several disadvantages. First, it increases the cost of production because the correlation process takes up labor and machine time that can otherwise be used for production. Running correlation is a labor-intensive task because the comparison of testing results and reference data has been done manually by an operator or engineer. Running correlation also requires machine time, which is proportionate to the number of dice that are sampled for testing. Thus, an inefficient sample size would increase the machine time required. An inefficient sample size can also reduce the life span of a correlation wafer as each wafer can only be tested for a limited number of times before becoming damaged; in turn, a shorten life span would require more labor and machine time to be spent to prepare another correlation wafer.
Second, the correlation results are sometimes inconsistent and unreliable. Often times, wafers are manufactured and tested by independent subcontractors at different sites and the subcontractor at each site may have developed different sets of correlation criteria. For example, subcontractors may have different correlation-passing requirement. A subcontractor may require 97% of the dice to have matching testing results while a different subcontractor may lower the requirement to 95%. As another example, some subcontractors do not treat a good die becoming bad die as non-matching because this change may be attributed to the normal wear and tear involved in wafer-testing; but some other subcontractors treat it as non-matching. Applying different sets of correlation criteria, a subcontractor may conclude that a defect exists in the testing equipments while another subcontractor at another site may conclude otherwise. The inconsistent correlation results may lead to more inaccurate wafer-testing results.
Because wafer correlation is an important diagnostic technique, there exists a need for an efficient and reliable correlation system and a method of using such correlation system. In particular, there is a need to minimize labor and machine time and turn out consistent correlation results.